Industrial control systems, process control systems, distributed control systems and the like in process industries often comprise both hard wired data networks and wireless data networks. Process industries can include branches of industry such as pulp and paper, pharmaceuticals, food production, oil and gas extraction, production, processing and the like. Wireless sensor networks are used to communicate measurements and some control data between wireless field devices such as sensors and the industrial control system. Wireless sensors are often battery powered.
Service life for a battery-powered wireless sensor depends on power use. Service life for a wireless sensor network also depends therefore on power use by individual wireless sensors. The radio receiver/transmitter of a wireless sensor typically consumes most power while transmitting and receiving, and power while listening for transmissions, and almost no power during an inactive state. Conventionally wireless sensors in a WSN are configured to use little or no energy during inactive periods, also called a sleep state, conserving energy for a limited number of periods for listening for signals and/or for transmitting a signal.
There are many different wireless protocols in use in industry generally. They include proprietary protocols, open protocol and networks with more than one type of protocol operating in the same broadcast/reception area. Two examples of wireless standards in use in industry are called ISA100 also including version ISA 100.11a, for wireless sensing in industrial automation applications, and WirelessHART. WirelessHART is a standard that has been developed to be compatible with older HART standards for communication between industrial devices.
WirelessHART [1] is used quite widely in industry, and is based on the wired HART Communication Protocol that has been on the market since the 1980s. WirelessHART is a standard developed for the industrial process automation and control automation. It is a simple, robust, reliable, secure, self-healing and self-organizing multi-hop wireless mesh network. The WirelessHART technology is fully compatible with the HART standard but WirelessHART has all the benefits of the wireless technology. WirelessHART uses radios compliant with IEEE 802.15.4-2006 [2]. WirelessHART is Time Division Multiple Access (TDMA) based where all the devices in the network are time synchronized. A pre-schedule with fixed time slots is used in order to reduce transmission collisions between devices. One cycle of slots is called a super frame in which the devices have their send, receive, retransmission and alternative path slots.
FIG. 3 (Prior Art) shows a data packet according to the WirelessHART standard which is divided into six layers. The Physical Layer 400 consists of a preamble, start flag (delimiter), and byte count, where the delimiter is used for training of a receivers radio. The Data Link Layer starts with a single byte value of 0x41 and the address specifier specifying 2 or 8 byte source and/or destination addresses. The Sequence Number and the network ID declares what network the device belongs to. If a device receives a packet from another network it will discard it. The Data Link Protocol Data Unit (DLPDU) 300 is the actual Data Link Layer packet and the DLPDU-specifier specifies: the packet priority, if the network key is used and the type of the packet. The data link layer ends with a Keyed Message Integrity Code (MIC) and a Cyclic Redundancy Check (CRC). FIG. 6 (Prior Art) shows a simple network operating with a WirelessHART standard. The network contains wireless field devices A-E and a standard gateway. The network also has a Network manager and a Security manager function.
Referring again to FIG. 2, the Network Layer 200 starts with a control byte specifying the size of the source and destination address, either 2 or 8 bytes. The control byte also specifies if Expanded Routing Information (ERI) is used. The forwarding devices decrement the TTL (time to live, eg 10 hops) and discards the packet if it becomes 0 except when it is 0xFF then it will not be decremented and is always forwarded onward by the devices. The ASN Snippet is the last 16 bits of the ASN when the Network Layer where invoked. The Graph ID is used to route the packet to the final destination and contains a list of devices which can be used.
The Security Sub-Layer ensures a secure communication between the sender and the final destination, an end-to-end type of security. The length of the Sub-Layer depends on the type of security used. The type of security used is specified in the Security Control Byte (SCB). Session keyed uses a 1 byte nonce Counter, and Join and handheld keyed use a 4 byte nonce counter. The Security Sub-Layer also includes a 4 byte MIC for the deciphering of the Network Layer payload.
The Transport Layer 100 is enciphered and contains a transport control byte which is used to indicate communication error or command response status. The extended device status is used to indicate the status of the device. The Application Layer consists of the actual command. It includes the command number, the data length and the actual data.
Minimizing the energy consumption of a battery powered device is very important. In general, to send one packet for each sensed data value is energy inefficient in most cases. In some cases data aggregation, aggregation of the raw data may be used. EP1626532, entitled Wireless building control architecture, assigned to Siemens Building Tech AG, describes building automation systems and in particular, a wireless building control architecture that implements automation of building systems in which collections or groups of sensors or actuator data are aggregated.
Data aggregation is used to increase the throughput in many data communication techniques. In, e.g., IEEE 802.11e and 802.11n frame aggregation is used to increase the throughput by sending two or more data frames in a single transmission [5]. Two different types of frame aggregation are defined in the IEEE 802.11n standard: MAC Service Data Unit (MSDU) aggregation and Message Protocol Data Unit (MPDU). For Wireless Sensor Network (WSN) there exist conference papers that mention aggregating data in a network. For example Krishnamachari et al, in a paper entitled Impact of Data Aggregation in Wireless Sensor Networks, B. Krishnamachari, D. Estrin, and S. B. Wicker, in Proc. 2d International Conference on Distributed Computing Systems Workshop, 2002, describe that a WSN can benefit in terms of energy savings by utilizing data aggregation. The article focuses on a model of data-centric routing in contrast to the end-to-end routing used in wireless sensor networks. It compares the performance of data-centric routing with traditional end-to-end routing schemes. Nevertheless, their work focuses more on routing than how to actually perform data aggregation. EP 1 538 806 A1 entitled Method of frame aggregation, assigned to Lucent Inc., describes a method for dynamically aggregating frames for transmission of voice data to take advantage of variations in available bandwidth in a channel to transmit an increased amount of data within a restricted bandwidth. Frame aggregation for cellular networks has been described, where the physical layer is formed by dynamically varying an aggregate packet size depending on the channel characteristics used for communicating voice and/or data. The physical layer frame may correspond with a payload having at least two content frames and at least one IP header. The above approach is used to address problems with bandwidth and voice frames.
However, data packet aggregation is not supported in all wireless protocols suitable for industrial use. For example the above described version of the WirelessHART standard does not define a data packet which is constructed to facilitate data aggregation between different devices. This means that for each item of data generated by a wireless field device in a conventional wireless network the transmitted packet will have to travel all the way from the device to the destination, regardless of the size of the data. All these transmissions consume a large amount of energy in wireless devices with a limited energy supply. In addition the requirement for security of the data being communicated means that the data is encrypted and can only be read by the sending or the receiving node, ie the data is protected by end-to-end security.